This invention relates to single or multilayer smart susceptors used for achieving thermal uniformity in induction processing of organic matrix composites or metals. The improved smart susceptor has a coating to minimize oxidation during repeated use in induction process manufacturing.
An induction processing system used for fabrication of material combines, metals and the like are disclosed in U.S. Pat. Ser. Nos. 5,808,281, 5,728,309, and 5,645,744 and is hereby incorporated by reference. The induction processing system uses susceptors to translate electrical energy to heat energy for fabrication of various parts and structures. The susceptors are often referred to as smart susceptors because the material composition is specifically chosen to produce a set temperature point when used in an induction processing system.
The sheets of material used to construct the susceptor may consist of ferromagnetic materials including a combination of iron (Fe), nickel (Ni), and/or cobalt (Co). For the higher temperature applications the alloys may be Co based with additions of Fe and Ni. Each specific composition and combination of material sheets constructed is based on the Curie temperature characteristics desired of the susceptor. The Curie Point at which there is a transition between the ferromagnetic and paramagnetic phases of a material is used to set the equilibrium temperature point caused by inductive heating in the inductive processing system.
The use of smart susceptor alloys such as combinations of Co, Ni and Fe material in cycling to elevated temperatures in inductive processing systems may cause oxidation of the susceptor material. It has been found that at elevated temperatures these alloys may oxidize at a relatively aggressive rate. In addition, the iron oxide formed may create a low melting point intermetallic interface when positioned in close proximity to the die material at elevated temperature. These conditions may cause the susceptors to deteriorate prematurely requiring replacement thereof. Also, the susceptors may interact with the die material, a ceramic, which necessitates die repair or replacement.
The known approach to reducing the effects of the oxidation of the susceptor has been to insert an intermediate metal sheet of material, such as Inconel 625(trademark), between the susceptor and the die. Material such as Inconel 625(trademark) is a nickel base alloy that is non-magnetic and has oxidation resistive qualities. While this solves the interaction problem, it does not eliminate the oxidation problem and adds additional parts to the system tooling requirements.
One solution to oxidation of the metal surface is the coating thereof. Examples include coating of steel and other materials in internal combustion engines where the high temperature combined with exposure to air create an environment that may be corrosive and cause erosion of metal surfaces. A method of flame or plasma spraying a thin coating layer on the exposed metal surfaces has been used to reduce oxidation damage. Various materials have been used to coat metals and other materials depending on the application. U.S. Pat. No. 3,762,884 discloses one such process as well as surveying other methods. These coatings were not applied to environments such as those for which the present invention is intended nor have coating compositions been formulated to solve the smart susceptor problem.
The coated smart susceptor and method therefore improves the control of superplastic forming, hot forming and/or heat treating processes to achieve a higher quality fabricated part. The coated susceptor use also enables cold loading and unloading of parts that improves dimensional control. The quick thermal cycles with induction processing and the improved life of the susceptor due to the coating reduces the need for batch loading of material as such is no longer required for economic viability of the fabrication processes.
As can be seen, there is a need for a simple, effective protectant to minimize oxidation of susceptors used in induction processing systems.
An improved susceptor construction and method according to the present invention comprises a metal alloy composition susceptor having a nickel aluminide (NiAl) coating thereon.
In one aspect of the present invention, a susceptor for temperature control of a part fabrication in an induction processing system comprises a single layer or a lamination of layers of ferromagnetic material susceptible to heating by induction, each having a selected Curie point. The single layer or laminated material layers are then coated with a nickel aluminide surface coat for oxidation protection during temperature cycling for part fabrication in the induction processing system. Also, the temperature at which the nickel aluminide may be sprayed to coat a susceptor and any exothermic reaction from the process generates heat creating a sintering effect to bond the coating to the susceptor outer surface.
In another aspect of the invention, the multiplayer susceptor may include an Al2O3 adherent layer formed on the surface coating to inhibit deterioration of the surface coating. The nickel aluminide coating may create it""s own Al2O3 layer at the surface. This oxide film and the very stable thermally resistant nickel aluminide layer underneath may form an oxidation barrier.
A further aspect of the present invention involves the method for producing a single or multilayer susceptor having a surface coat oxidation protective layer comprising the steps of fabricating a single layer or a plurality of laminated material layers of ferromagnetic material, introducing a wire or powder mixture of nickel and aluminum into a flame or plasma spray gun, operating the spray gun to heat the powder or wire to form droplets and to spray the droplets on an outer surface of the single or laminated material layers, and continuing the spraying to sinter a surface coating of nickel aluminide to the outer surface.